Switchable Lens

ABSTRACT

A display apparatus has a switchable birefringent lens array. The display apparatus produces a substantially linearly polarised output. The lens array comprises birefringent material arranged between a planar surface of a first substrate and a relief substrate of a second substrate defining an array of cylindrical lenses. The lens array has electrodes for applying a control voltage across the birefringent material for electrically switching the birefringent material between a first mode and a second mode. In the first mode the lens array modifies the directional distribution of incident light polarised in a predetermined direction. In the second mode the lens array has substantially no effect on incident light polarised in said predetermined direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.10/544,330, which is a national phase application of PCT/GB04/00374,filed Sep. 16, 2005, which PCT application claimed priority to GreatBritain application GB 0302658, filed Feb. 5, 2003.

BACKGROUND

1. Field of Invention

The present invention relates to optical display apparatus, inparticular switchable active lenses for display apparatus. Such aswitchable active lens may be used to provide: a switchable twodimensional (2D)/three dimensional (3D) autostereoscopic displayapparatus; a switchable high brightness reflective display system; or amulti-user display system. Such display systems may be used in computermonitors, telecommunications handsets, digital cameras, laptop anddesktop computers, games apparatuses, automotive and other mobiledisplay applications.

2. Description of Related Art

3D Displays

Normal human vision is stereoscopic, that is each eye sees a slightlydifferent image of the world. The brain fuses the two images (referredto as the stereo pair) to give the sensation of depth. Three dimensionalstereoscopic displays replay a separate, generally planar, image to eachof the eyes corresponding to that which would be seen if viewing a realworld scene. The brain again fuses the stereo pair to give theappearance of depth in the image.

FIG. 1 a shows in plan view a display surface in a display plane 1. Aright eye 2 views a right eye homologous image point 3 on the displayplane and a left eye 4 views a left eye homologous point 5 on thedisplay plane to produce an apparent image point 6 perceived by the userbehind the screen plane.

FIG. 1 b shows in plan view a display surface in a display plane 1. Aright eye 2 views a right eye homologous image point 7 on the displayplane and a left eye 4 views a left eye homologous point 8 on thedisplay plane to produce an apparent image point 9 in front of thescreen plane.

FIG. 1 c shows the appearance of the left eye image 10 and right eyeimage 11. The homologous point 5 in the left eye image 10 is positionedon a reference line 12. The corresponding homologous point 3 in theright eye image 11 is at a different relative position 3 with respect tothe reference line 12. The separation 13 of the point 3 from thereference line 12 is called the disparity and in this case is a positivedisparity for points which will lie behind the screen plane.

For a generalised point in the scene there is a corresponding point ineach image of the stereo pair as shown in FIG. 1 a. These points aretermed the homologous points. The relative separation of the homologouspoints between the two images is termed the disparity; points with zerodisparity correspond to points at the depth plane of the display. FIG. 1b shows that points with uncrossed disparity appear behind the displayand FIG. 1 c shows that points with crossed disparity appear in front ofthe display. The magnitude of the separation of the homologous points,the distance to the observer, and the observer's interocular separationgives the amount of depth perceived on the display.

Stereoscopic type displays are well known in the prior art and refer todisplays in which some kind of viewing aid is worn by the user tosubstantially separate the views sent to the left and right eyes. Forexample, the viewing aid may be colour filters in which the images arecolour coded (e.g. red and green); polarising glasses in which theimages are encoded in orthogonal polarisation states; or shutter glassesin which the views are encoded as a temporal sequence of images insynchronisation with the opening of the shutters of the glasses.

Autostereoscopic displays operate without viewing aids worn by theobserver. In autostereoscopic displays, each of the views can be seenfrom a limited region in space as illustrated in FIG. 2.

FIG. 2 a shows a display device 16 with an attached parallax opticalelement 17. The display device produces a right eye image 18 for theright eye channel. The parallax optical element 17 directs light in adirection shown by the arrow 19 to produce a right eye viewing window 20in the region in front of the display. An observer places their righteye 22 at the position of the window 20. The position of the left eyeviewing window 24 is shown for reference. The viewing window 20 may alsobe referred to as a vertically extended optical pupil.

FIG. 2 b shows the left eye optical system. The display device 16produces a left eye image 26 for the left eye channel. The parallaxoptical element 17 directs light in a direction shown by the arrow 28 toproduce a left eye viewing window 30 in the region in front of thedisplay. An observer places their left eye 32 at the position of thewindow 30. The position of the right eye viewing window 20 is shown forreference.

The system comprises a display and an optical steering mechanism. Thelight from the left image 26 is sent to a limited region in front of thedisplay, referred to as the viewing window 30. If an eye 32 is placed atthe position of the viewing window 30 then the observer sees theappropriate image 26 across the whole of the display 16. Similarly theoptical system sends the light intended for the right image 18 to aseparate window 20. If the observer places their right eye 22 in thatwindow then the right eye image will be seen across the whole of thedisplay. Generally, the light from either image may be considered tohave been optically steered (i.e. directed) into a respectivedirectional distribution.

FIG. 3 shows in plan view a display device 16,17 in a display plane 34producing the left eye viewing windows 36,37,38 and right eye viewingwindows 39,40,41 in the window plane 42. The separation of the windowplane from the display is termed the nominal viewing distance 43. Thewindows 37,40 in the central position with respect to the display are inthe zeroth lobe 44. Windows 36,39 to the right of the zeroth lobe 44 arein the +1 lobe 46, while windows 38,41 to the left of the zeroth lobeare in the −1 lobe 48.

The viewing window plane of the display represents the distance from thedisplay at which the lateral viewing freedom is greatest. For pointsaway from the window plane, there is a diamond shaped autostereoscopicviewing zone, as illustrated in plan view in FIG. 3. As can be seen, thelight from each of the points across the display is beamed in a cone offinite width to the viewing windows. The width of the cone may bedefined as the angular width.

If an eye is placed in each of a pair viewing zones such as 37,40 thenan autostereoscopic image will be seen across the whole area of thedisplay. To a first order, the longitudinal viewing freedom of thedisplay is determined by the length of these viewing zones.

The variation in intensity 50 across the window plane of a display(constituting one tangible form of a directional distribution of thelight) is shown with respect to position 51 for idealised windows inFIG. 4 a. The right eye window position intensity distribution 52corresponds to the window 41 in FIG. 3, and intensity distribution 53corresponds to the window 37, intensity distribution 54 corresponds tothe window 40 and intensity distribution 55 corresponds to the window36.

FIG. 4 b shows the intensity distribution with position schematicallyfor more realistic windows. The right eye window position intensitydistribution 56 corresponds to the window 41 in FIG. 3, and intensitydistribution 57 corresponds to the window 37, intensity distribution 58corresponds to the window 40 and intensity distribution 59 correspondsto the window 36.

The quality of the separation of images and the extent of the lateraland longitudinal viewing freedom of the display is determined by thewindow quality, as illustrated in FIG. 4. FIG. 4 a shows the idealviewing windows while FIG. 4 b is a schematic of the actual viewingwindows that may be outputted from the display. Several artefacts canoccur due to inadequate window performance. Cross talk occurs when lightfrom the right eye image is seen by the left eye and vice versa. This isa significant 3D image degradation mechanism which can lead to visualstrain for the user. Additionally, poor window quality will lead to areduction in the viewing freedom of the observer. The optical system isdesigned to optimised the performance of the viewing windows.

The parallax optical element may be a parallax barrier. The displaycomprises a backlight, an array of electronically adjustable pixels(known as a Spatial Light Modulator, SLM) arranged in columns and rowsand a parallax barrier attached to the front of the display asillustrated in plan view in FIG. 5.

Parallax barriers rely on blocking the light from regions of the displayand therefore reduce the brightness and device efficiency, generally toapproximately 20-40% of the original display brightness. Parallaxbarriers are not readily removed and replaced due to the requirements ofsub-pixel alignment tolerances of the barrier with respect to the pixelstructure of the display in order to optimise the viewing freedom of thedisplay. The 2D mode is half resolution.

Another type of parallax optic (cf. parallax barriers) well known in theart for use in stereoscopic displays is called the lenticular screen,which is an array of vertically extended cylindrical microlenses. Theterm “cylindrical” as used herein has its normal meaning in the art andincludes not only strictly spherical lens shapes but also asphericallens shapes. The pitch of the lenses corresponds to the viewpointcorrection condition, that is the pitch of the parallax barrier isslightly smaller than twice the pitch of the pixel array in order tosteer the light from each pixel to the viewing window. In such adisplay, the resolution of each of the stereo pair images is half thehorizontal resolution of the base LCD, and two views are created.

The curvature of the lenses is set substantially so as to produce animage of the LCD pixels at the window plane. As the lenses collect thelight in a cone from the pixel and distribute it to the windows,lenticular displays have the full brightness of the base panel.

FIG. 6 shows a typical structure for a lenticular display device using alenticular array. A backlight 60 produces a light output 62 which isincident on an LCD input polariser 64. The light is transmitted througha TFT LCD substrate 66 and is incident on a repeating array of pixelsarranged in columns and rows in an LCD pixel plane 67. The red pixels68,71,73, green pixels 69,72,75 and blue pixels 70,73 each comprise anindividually controllable liquid crystal layer and are separated byregions of an opaque mask called a black mask 76. Each pixel comprises atransmissive region, or pixel aperture 78. Light passing through thepixel is modulated in phase by the liquid crystal material in the LCDpixel plane 74 and in colour by a colour filter positioned on an LCDcolour filter substrate 80. The light then passes through an outputpolariser 82 after which is placed a parallax barrier 84 and a parallaxbarrier substrate 86. The parallax barrier 84 comprises an array ofvertically extended transmissive regions separated by verticallyextended opaque regions and serves to direct light from alternate pixelcolumns 69,71,73,75 to the right eye as shown by the ray 88 for lightfrom pixel 69 and from the intermediate columns 68,70,72,74 to the lefteye as shown by the ray 90 (this overall light direction pattern forminganother example of a directional distribution of light). The observersees the light from the underlying pixel illuminating the aperture ofthe barrier, 92. The light then passes through a lenticular screensubstrate 94 and a lenticular screen 96 which is formed on the surfaceof the lenticular screen substrate 92. As for the parallax barrier, thelenticular screen 94 serves to direct light from alternate pixel columns69,71,73,75 to the right eye as shown by the ray 88 from the pixel 69and from the intermediate columns 68,70,72,74 to the left eye as shownby the ray 90 from pixel 68. The observer sees the light from theunderlying pixel illuminating the aperture of the individual lenticule,98 of the lenticular screen 96. The extent of the captured light cone isshown by the captured rays 100.

Lenticular displays are described in T. Okoshi “Three DimensionalImaging Techniques”, Academic Press, 1976. One type of lenticulardisplay using a spatial light modulator is described in U.S. Pat. No.4,959,641, in particular non-switching lenticular elements in air.

A lenticular display using cylindrical lenses that are tilted withrespect to columns of pixels of a display is described in “multiview3D-LCD” published in SPIE Proceedings Vol. 2653, 1996, pages 32 to 39.

The viewing freedom of the flat panel displays described above islimited by the window structure of the display.

A display in which the viewing freedom is enhanced by measuring theposition of an observer and moving the parallax element incorrespondence is described in EP-0,829,743. Such an observermeasurement apparatus and mechanical actuation is expensive and complex.

A display in which the window optical structure is not varied (a fixedparallax optic display for example) and the image data is switched incorrespondence to the measured position of the observer such that theobserver maintains a substantially orthoscopic image is described forexample in EP-0,721,131.

As described above, the use of parallax optics to generate a spatiallymultiplexed 3D display limits the resolution of each image to at besthalf of the full display resolution. In many applications, the displayis intended to be used for a fraction of the time in the 3D mode, and isrequired to have a full resolution artefact free 2D mode.

One type of display in which the effect of the parallax optic is removedis Proc. SPIE vol. 1915 Stereoscopic Displays and Applications IV (1993)pp 177-186, “Developments in Autostereoscopic Technology at DimensionTechnologies Inc.”, 1993. In this case, a switchable diffuser element isplaced in the optical system used to form the light lines. Such aswitchable diffuser could be for example of the Polymer Dispersed LiquidCrystal type in which the molecular arrangement switches between ascattering and non-scattering mode on the application of an appliedvoltage across the material. In the 3D mode, the diff-user is clear andlight lines are produced to create the rear parallax barrier effect. Inthe 2D mode, the diffuser is scattering and the light lines are washedout, creating the effect of a uniform light source. In this way, theoutput of the display is substantially Lambertian and the windows arewashed out. An observer will then see the display as a full resolution2D display. Such a display suffers from Fresnel diffraction artefacts inthe 3D mode, as well as from unwanted residual scatter in the diffuser'sclear state which will increase the display cross-talk. Therefore, sucha display is likely to exhibit higher levels of visual strain.

In another type of switchable 2D-3D display disclosed in EP-0,833,183, asecond LCD is placed in front of the display to serve as a parallaxoptic. In a first mode, the parallax LCD is clear so that no windows areproduced and an image is seen in 2D. In a second mode, the device isswitched so as to produce slits of a parallax barrier. Output windowsare then created and the image appears to be 3D. Such a display hasincreased cost and complexity due to the use of two LCD elements as wellas being of reduced brightness or having increased power consumption. Ifused in a reflective mode 3D display system, parallax barriers result invery poor brightness due to attenuation of light by the blocking regionsof the parallax barrier both on the way in and out of the display.

In another type of switchable 2D-3D display disclosed in EP-0,829,744, aparallax barrier comprises a patterned array of halfwave retarderelements. The pattern of retarder elements corresponds to the pattern ofbarrier slits and absorbing regions in a parallax barrier element. In a3D mode of operation, a polariser is added to the display so as toanalyse the slits of the patterned retarder. In this way, an absorbingparallax barrier is produced. In the 2D mode of operation, the polariseris completely removed as there is no involvement of any polarisationcharacteristics in the 2D mode of operation. Thus the output of thedisplay is full resolution and full brightness. One disadvantage is thatsuch a display uses parallax barrier technology and thus is limited toperhaps 20-30% brightness in the 3D mode of operation. Also, the displaywill have a viewing freedom and cross talk which is limited by thediffraction from the apertures of the barrier.

It is known to provide electrically switchable birefringent lenses forpurposes of switching light directionally. It is known to use suchlenses to switch a display between a 2D mode of operation and a 3D modeof operation.

For example, electrically switchable birefringent liquid crystalmicrolenses are described in European Optical Society Topical MeetingsDigest Series: 13, 15-16 May 1997 L. G. Commander et al “Electrodedesigns for tunable microlenses” pp 48-58.

In another type of switchable 2D-3D display disclosed in U.S. Pat. No.6,069,650 and WO-98/21620, switchable microlenses comprising alenticular screen filled with liquid crystal material are used to changethe optical power of a lenticular screen. U.S. Pat. No. 6,069,650 andWO-98/21620 teach the use of an electro-optic material in a lenticularscreen whose refractive index is switchable by selective application ofan electric potential between a first value whereby the light outputdirecting action of the lenticular means is provided and a second valuewhereby the light output directing action is removed.

A 3D display comprising a liquid crystal Fresnel lens is described in S.Suyama et al “3D Display System with Dual Frequency Liquid CrystalVarifocal Lens”, SID 97 DIGEST pp 273-276.

SUMMARY

In a first aspect the present invention provides for polarisationmatching between the output of a display device and the alignmentdirection in an active lens.

In one form of the first aspect of the present invention, there isprovided a switchable birefringent lens array for a display apparatusproducing a substantially linearly polarised output, the lens arraycomprising:

birefringent material arranged between a planar surface and a reliefsurface defining an array of cylindrical lenses; and

electrodes for applying a control voltage across the birefringentmaterial for electrically switching the birefringent material between afirst mode and a second mode, the lens array being arranged in saidfirst mode to modify the directional distribution of incident lightpolarised in a predetermined direction and in said second mode to havesubstantially no effect on incident light polarised in saidpredetermined direction,

wherein:

in the first mode, at said relief surface the birefringent material isaligned parallel to the geometrical axis of the cylindrical lenses; and

in the first mode, at said planar surface the birefringent material isaligned parallel to the planar surface at a predetermined angle suchthat in the first mode, the alignment direction twists between theplanar surface and the relief surface.

It is advantageous for the alignment direction of the birefringentmaterial to be parallel to the geometric axis of the cylindrical lenses,because this avoids dislocations at the relief surface due tocompetition between the alignment layer surface energy and the surfacerelief structure alignment surface energy, which dislocations mightcause scatter, increase optical crosstalk, reduce lens contrast and/orincrease relaxation times. It also simplifies manufacture, allowing theuse of known manufacturing techniques.

A lens array according to the first aspect the present invention has theunexpected advantage that substantially achromatic polarisation guidingwill take place in the birefringent material because of their relativelyhigh optical thickness in practical systems. In other words thepolarisation direction rotates as the light passes through thebirefringent material. This guiding effect can be used to control thepolarisation of the device in the lens array.

Such rotation of the polarisation direction by the lens array means thatno additional waveplate is required between the display and the activelens (although optionally one or more waveplates may be added), thusallowing the viewing distance of the element to be reduced in the firstmode of operation and the device cost to be reduced.

The alignment may be provided by any suitable means, for examplealignment layers.

In another form of the first aspect of the present invention, there isprovided a directional display apparatus comprising:

a substantially linearly polarised output display device; and

a switchable birefringent cylindrical lens arranged in a first mode tomodify the directional distribution of the polarised output displaydevice and in a second mode to substantially cause no modification ofthe directional distribution of the display device comprising:

-   -   a surface relief layer defining a cylindrical microlens array,    -   an alignment layers formed on the surface relief layer such that        the alignment of the birefringent material at the surface relief        surface the first mode of operation is substantially parallel to        the geometric axis of the cylindrical lenses; and    -   electrode layers arranged to switch the orientation of the        birefringent material between at least a first and second        orientation for first and second modes respectively,    -   the alignment of the birefringent material at the planar        substrate being aligned in cooperation with the output        polarisation of the display device so that in the first mode of        operation the polarisation is transmitted through the        birefringent material with a twist to be substantially parallel        to the geometric axis of the cylindrical microlens array at the        surface relief surface.

Preferably, one or more of the following optional features are present:

-   -   the birefringent material is a liquid crystal.    -   the alignment direction at the planar substrate is parallel or        orthogonal to the output polarisation of the display device.    -   the substantially polarised display may comprise a partially        polarised or unpolarised display and a polariser element.    -   the display may be a spatial light modulator for non-display        directionality switching applications.    -   there is a twist of the polarisation state passing through the        lens in the first mode.    -   the twist of the polarisation state is 45 degrees, −45 degrees        or 135 degrees    -   additional waveplates may be incorporated between the display        device and the active lens to rotate the output polarisation of        the substantially polarised display.

Thus, hereinafter described embodiments of the present invention provideone or more of the following advantages:

-   -   The alignment direction of the liquid crystal can be parallel to        the geometric axis of the surface relief cylindrical        microlenses.    -   The lens surface is convenient to manufacture with known surface        alignment techniques.    -   No additional waveplate is required between the display and the        active lens, thus the viewing distance of the element can be        reduced in the 3D mode of operation and the device cost reduced.    -   Disclinations will not be encountered in the switching lens due        to competition between the alignment layer surface energy and        the surface relief structure alignment surface energy.    -   The contrast of the active lens cell will be optimized.    -   The switching response time is minimized.    -   Degeneracy in the lens element is minimized, so that the device        switches uniformly.    -   The invention has the unexpected advantage that polarisation        guiding will take place in the lenses because of their        relatively high optical thickness in practical systems. This        guiding effect can be used to control the polarisation of the        device in the active lens.    -   The display can produce high brightness in 2D and 3D modes with        a fixed liquid crystal display output polarisation state.

In a second aspect, the present invention provides for lens arrayshaving homeotropically aligned birefringent material.

In one form of the second aspect of the present invention, there isprovided a switchable birefringent lens array for a display apparatusproducing a substantially linearly polarised output, the lens arraycomprising:

birefringent material arranged between a surface at least one of whichis a relief surface defining an array of cylindrical lenses; and

electrodes for applying a control voltage across the birefringentmaterial for electrically switching the birefringent material between afirst mode and a second mode, the lens array being arranged in saidfirst mode to modify the directional distribution of incident lightpolarised in a predetermined direction and in said second mode to havesubstantially no effect on incident light polarised in saidpredetermined direction, wherein the birefringent material ishomeotropically aligned at said at least one relief surface.

Such a lens array has the advantage that it may be configured so as notrequire any power consumption in the second mode of operation. This isbecause in the absence of a control voltage the birefringent material isaligned parallel to the optical axis, whereby the light experiences theordinary refractive index of the birefringent material at the reliefsurface, which most conveniently is the second mode of operation inwhich there is substantially no effect on the incident light.

For example, in a display device switchable between a 2D mode and anautostereoscopic 3D mode, this means that the 2D mode does not requirepower consumption. Therefore 2D operating time on batteries would beunaffected. In an active lens, because lens depths tend to be of theorder of tens of microns, the voltage that needs to be applied to switchthe thick parts of the cell is substantially in excess of the standardliquid crystal cell operating voltages, for example 5V for 5 μm thickcells. Therefore the power consumption of the liquid crystal in thedriven mode for this kind of lens cell is significantly higher than thepower consumption of a standard 5 μm thick LCD using the same liquidcrystal and driving frequency. Therefore it is undesirable to have a 2Ddriven mode. If the lens switch is damaged, the default mode ofoperation is in the 2D mode, and so no degradation to the image will beseen. The focal length of the lenses can be tuned in the 3D mode bymodifying the voltage to suit the desired window appearance.

The alignment may be provided by any suitable means, for example analignment layer. Homeotropic alignment layers allow the use of readilyavailable polymer materials to form the lens surface without excessivelyhigh refractive indices. Such polymer materials do not suffer from highcost, high toxicity and difficult processing regimes.

In another form of the second aspect of the present invention, there isprovided an optical switching apparatus comprising a switchablebirefringent lens comprising a birefringent optical material and a firstsubstrate wherein:

a first homeotropic alignment layer is formed on the surface reliefstructure; and

the dielectric anisotropy of the birefringent material is less thanzero, such that the switchable lens operates in the first mode when anelectric field is applied to the cell and in a second mode when noelectric field is applied to the cell. Preferably, one or more of thefollowing optional features are present:

-   -   in the first mode, the alignment of the birefringent material        optical axis at the surface relief structure is substantially        parallel to the geometric microlens axis.    -   which alignment is provided by a homogeneous bias of the        alignment layer    -   which alignment is provided by a homogeneous bias provided by a        grooved structure    -   the alignment at the planar substrate        -   is homeotropic        -   is homogeneous;        -   comprises homeotropic and homogenous alignment such that it            shows homogeneous alignment properties in a first mode and            homeotropic alignment properties in a second mode.        -   provides a twist of the incident polarisation state in the            first mode of operation such that the polarisation state at            the surface relief structure is parallel to the birefringent            lens optical axis.    -   a waveplate is positioned between the display device and the        planar substrate such that the polarisation angle at the planar        substrate is parallel to the polarisation angle at the surface        relief substrate.

In one form of the third aspect of the present invention, there isprovided a display apparatus comprising:

a display device having a spatial light modulator and an outputpolariser; and

an electrically switchable birefringent lens array arranged to receivelight from the spatial light modulator,

wherein the lens array is arranged between the spatial light modulatorand the output polariser of the display device.

Such a display device provides a reduced viewing distance as compared tothe output polariser being arranged immediately on the output side ofthe spatial light modulator which is its normal position. This isbecause the absence of the output polariser between the spatial lightmodulator and the lens array allows the distance therebetween to bereduced.

The third aspect of the invention is applicable to any type of displaydevice including a spatial light modulator and an output polariser. Itis particularly advantageous for a spatial light modulator, such as aliquid crystal modulator, which is arranged to phase modulate thepolarisation of the incident light so as to cause ellipticity androtation of the major axis of polarisation of light at each pixel by amodulated amount, the output polariser being, used to select lightpolarised in a predetermined direction, whereby the amount ofellipticity and rotation modulates the amplitude of the output light.However, the third aspect may alternatively be applied to an emissivespatial light modulator, for example which produces an unpolarisedoutput, the output polariser being used to polarise the output, or whichproduces a polarised output, the output polariser being a clean-uppolariser.

In another form of the third aspect of the invention, there is provideda directional display apparatus comprising:

a substantially linearly polarised output display device

an active lens comprising switchable birefringent cylindrical lensarranged in a first mode to modify the directional distribution of thepolarised output display device and in a second mode to substantiallycause no modification of the directional distribution of the displaydevice,

where the active lens is positioned between the pixel plane and anoutput polariser of the display device.

In one form of the fourth aspect of the present invention, there isprovided a display apparatus comprising:

an emissive spatial light modulator which is arranged to output lightwhich is substantially linearly polarised in each pixel of the spatiallight modulator; and

an electrically switchable birefringent lens array arranged to receivelight from the spatial light modulator.

In another form of the fourth aspect of the invention, there is providedan emissive light direction switching apparatus, comprising:

an optical switching apparatus comprising a switchable birefringent lenscomprising a switchable birefringent optical material; and

an emissive spatial light modulator apparatus comprising an array ofemitting pixel regions each with a substantially polarised opticaloutput.

Such a display apparatus can be configured with high optical efficiency.

In one form of a fifth aspect of the present invention, there isprovided an active birefringent lens array apparatus for a displayapparatus comprising:

a birefringent material and a substantially isotropic material havingrelief surface therebetween defining an array of lenses;

electrodes for applying a control voltage across the birefringentmaterial for electrically switching the birefringent material between afirst mode and a second mode, the lens array being arranged in saidfirst mode to modify the directional distribution of incident lightpolarised in a predetermined direction and in said second mode to havesubstantially no effect on incident light polarised in saidpredetermined direction; and

a voltage controller for controlling the voltage across the electrodesto switch between the first and second mode, the voltage controllerbeing arranged to adjust the voltage applied in the first and secondmode to compensate for variations in the temperature of the lens arrayapparatus.

The refractive index of the isotropic material and also the ordinary andextraordinary refractive indices of the birefringent material vary withtemperature. Thus a lens array designed to operate at one temperaturemay have poor optical properties at another temperature. However,voltage control allows for compensation of the variations withtemperature. This improves the optical performance and allows operationof the lens array across a broader operation temperature range.

In one type of apparatus, a temperature sensor is used to sense thetemperature of the lens array apparatus, and the voltage controller isarranged to adjust the voltage applied in the first and second mode tocompensate for variations in the temperature of the lens array apparatusin response to the temperature sensed by the temperature sensor.

This type of apparatus has the advantage of providing automaticcompensation for temperature changes.

In another type of apparatus, an input device allows a user to input avoltage adjustment, and the voltage controller being arranged to adjustthe voltage applied in the first and second mode in response to thevoltage adjustment input to the input device.

This type of apparatus has the advantage of simplicity.

Advantageously, the refractive index of the isotropic material isbetween the ordinary refractive index of the birefringent material andthe extraordinary refractive index of the birefringent material attemperatures below a limit of at least 25° C.

As compared to the typical situation that the refractive index of theisotropic material is equal the ordinary refractive index of thebirefringent material (or in other material systems, the extraordinaryrefractive index of the birefringent material) at the typical designtemperature of 20° C., the range of temperatures where the refractiveindex of the isotropic material is above the ordinary refractive indexof the birefringent material (or below the extraordinary refractiveindex of the birefringent material) is increased. However, in this rangethe voltage controller can compensate for the variations withtemperature by adjusting the effective refractive index of thebirefringent material, so this feature effectively increases theoperating temperature range. The limit may be greater than 25° C. tofurther increase the range.

Advantageously, the active birefringent lens array apparatus is used indisplay apparatus further comprising a spatial light modulator arrangedin series with the active birefringent lens array apparatus, wherein, attemperatures below a limit of at least 25° C., the power of the array oflenses in the first mode is greater than the power required for thearray of lenses to provide the best focus of the spatial lightmodulator.

As compared to the typical situation that the power of the array oflenses in the first mode is designed to provide best focus at thetypical design temperature of 20° C., the range of temperatures wherepower of the array of lenses in the first mode is too strong for bestfocus is increased. However, in this range, the voltage controller canreduce the power to adjust the focus of the lens array towards bestfocus by adjusting the effective refractive index of the birefringentmaterial, so this feature effectively increases the operatingtemperature range. The limit may be greater than 25° C. to furtherincrease the range.

In another form of a fifth aspect of the present invention, there isprovided active birefringent lens array apparatus for a displayapparatus comprising:

a birefringent material and a substantially isotropic material havingrelief surface therebetween defining an array of lenses;

electrodes for applying a control voltage across the birefringentmaterial for electrically switching the birefringent material between afirst mode and a second mode, the lens array being arranged in saidfirst mode to modify the directional distribution of incident lightpolarised in a predetermined direction and in said second mode to havesubstantially no effect on incident light polarised in saidpredetermined direction, wherein the refractive index of the isotropicmaterial is equal to one of the ordinary refractive index of thebirefringent material or the extraordinary refractive index of thebirefringent material at a temperature above 20° C. or at a temperatureat or above 25° C.

20° C. is a typical room temperature which is commonly used as a designtemperature for a display apparatus. Depending on the materials, therefractive index of the isotropic material is substantially equal to oneof the ordinary refractive index of the birefringent material or theextraordinary refractive index of the birefringent material. Thetemperature at which the refractive index of the isotropic material isexactly equal to that one of the refractive indices of the birefringentmaterial can be selected as a design parameter by choice of thematerials. In this form of a fifth aspect of the present invention, thattemperature is not the normal design temperature of 20° C. but ishigher, typically being at a temperature at or above 25° C. Such achoice is based on an appreciation that display apparatuses are usedmore often at temperatures above the normal design temperature of 20° C.Therefore raising the temperature at which the refractive index of theisotropic material is exactly equal to the relevant one of therefractive indices of the birefringent material actually causes therefractive index of the isotropic material to be closer to the relevantone of the refractive indices of the birefringent material over agreater proportion of the typical use of a display apparatus.

With currently preferred materials, the refractive index of theisotropic material is substantially equal to the ordinary refractiveindex of the birefringent material, and exactly equal at a temperatureabove 20° C., but with other materials the refractive index of theisotropic material may be substantially equal to the extraordinaryrefractive index of the birefringent material, and exactly equal at atemperature above 20° C.

Any or all of the various aspects of the invention may advantageously beused in combination. Therefore, in general, any of features of any ofthe aspects of the invention may be applied to any of the other aspectsof the invention.

In all the aspects of the present invention, the following commentsapply. The lens array may be used with any type of display apparatusproducing a substantially linearly polarised output, for example atransmissive spatial light modulator lit by a backlight having apolariser to polarise the output; or an emissive spatial light modulatorwhich may be intrinsically unpolarised and provided with a polariser ormay be polarised. In general, the display device may employ any type ofspatial light modulator to modulate the light of each pixel, includingtransmissive, emissive or reflective, or even a combination thereof.

The lens array is arranged in said first mode to modify the directionaldistribution of incident light. This may be used to achieve a variety ofdifferent effects including, but not limited to, the provision of: a 3Dautostereoscopic effect; regions of high brightness; or a multi-userdisplay system.

Thus such devices can be used for:

an autostereoscopic display means which can conveniently provide amoving full colour 3D stereoscopic image which may be viewed by theunaided eye in one mode of operation and a full resolution 2D image in asecond mode of operation;

a switchable high brightness transflective and reflective display systemwhich in a first mode may exhibit substantially non-directionalbrightness performance and in a second mode may exhibit substantiallydirectional brightness performance; or

a multi-viewer display means which can conveniently provide one movingfull colour 2D images to one observer and at least a second different 2Dimage to at least a second observer in one mode of operation and a fullresolution 2D image seen by all observers in a second mode of operation.

Embodiments of the present invention can provide the followingadvantages to provide the following advantages singly or in anycombination.

This invention enables the generation of autostereoscopic 3D images andfull resolution 2D images of high quality with low levels of image crosstalk and high brightness.

This invention also enables the generation of a directional multi-viewerdisplay that can be switched between a 2D mode and a mode in whichimages (which may be different) can be seen by different viewers from awide range of directions.

By arranging the microlenses to be internal to the glass substrate,reflections from the surfaces of the lenses can be minimised and theoutput surface (which may be planar) can be anti-reflection coated.

A high brightness transflective or reflective display advantageously hasa first mode with substantially non-directional properties as defined bythe reflector material of the display and in a second mode hasdirectional brightness property such that the display brightness isgreater from a defined range of angles. Such a display works in fullcolour and can be used to increase the brightness of both reflective andtransmissive modes of operation.

A multi-viewer display can be configured so that in one mode ofoperation all viewers can see the same image and in a second mode ofoperation different viewers can see different images to allow multiplesimultaneous uses of the display.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 a shows the generation of apparent depth in a 3D display for anobject behind the screen plane;

FIG. 1 b shows the generation of apparent depth in a 3D display for anobject in front of the screen plane;

FIG. 1 c shows the position of the corresponding homologous points oneach image of a stereo pair of images;

FIG. 2 a shows schematically the formation of the right eye viewingwindow in front of an autostereoscopic 3D display;

FIG. 2 b shows schematically the formation of the left eye viewingwindow in front of an autostereoscopic 3D display;

FIG. 3 shows in plan view the generation of viewing zones from theoutput cones of a 3D display;

FIG. 4 a shows the ideal window profile for an autostereoscopic display;

FIG. 4 b shows a schematic of the output profile of viewing windows froman autostereoscopic 3D display;

FIG. 5 shows the structure of a parallax barrier display;

FIG. 6 shows the structure of a lenticular screen display;

FIG. 7 a shows in a first cross section two cylindrical lenses of a lensarray in which the liquid crystal alignment is homogeneous at bothsurfaces of a lens;

FIG. 7 b shows the two cylindrical lenses of FIG. 7 a in a second crosssection perpendicular to that of FIG. 7 a;

FIG. 7 c shows in the same view as FIG. 7 b, an alternative arrangementin which the cusps are substantially in contact with the planar surfaceand an electrode is formed on the surface relief surface;

FIG. 8 a shows the alignment and polarisation directions for an activelens autostereoscopic 3D display;

FIG. 8 b shows the alignment and polarisation directions for an activelens brightness enhancing display;

FIG. 8 c shows the alignment and polarisation directions for a twistedactive lens autostereoscopic 3D display;

FIG. 8 d shows an alternative alignment and polarisation directions fora twisted active lens autostereoscopic 3D display;

FIG. 9 a shows in a first cross section two cylindrical lenses of a lensarray in which the liquid crystal alignment is homeotropic at bothsurfaces of a lens;

FIG. 9 b shows the two cylindrical lenses of FIG. 9 a in a second crosssection perpendicular to that of FIG. 9 a;

FIG. 10 a shows the alignment and polarisation directions for an activelens autostereoscopic 3D display incorporating homeotropic alignmentlayers;

FIG. 10 b shows the alignment and polarisation directions for an activelens autostereoscopic 3D display incorporating homeotropic andhomogeneous alignment layers;

FIG. 11 a shows in a first cross section two cylindrical lenses of alens array in which the liquid crystal alignment is homeotropic at onesurface and homogeneous at a second surface of an active lens;

FIG. 11 b shows the two cylindrical lenses of FIG. 11 a in a secondcross section perpendicular to that of FIG. 11 a;

FIG. 12 shows the alignment and polarisation directions for an activelens autostereoscopic 3D display incorporating homeotropic andhomogeneous alignment layers;

FIG. 13 a shows in a first cross section two cylindrical lenses of alens array in which the liquid crystal alignment can be homeotropic andhomogeneous at a lens surface;

FIG. 13 b shows the two cylindrical lenses of FIG. 13 a in a secondcross section perpendicular to that of FIG. 13 a;

FIG. 14 shows in cross section the structure of an active lensautostereoscopic 3D display with an internal polariser;

FIG. 15 shows in cross section the structure of an active lensautostereoscopic 3D display with an external polariser;

FIG. 16 shows the alignment and polarisation directions for an activelens autostereoscopic 3D display with an external polariser;

FIG. 17 shows an active lens autostereoscopic 3D display with apolarised emissive display;

FIG. 18 shows an active lens enhanced brightness reflective display;

FIG. 19 shows the alignment and polarisation directions for an activelens enhanced brightness reflective display;

FIG. 20 shows the alignment and polarisation directions for an activelens with tilted geometric lens axis;

FIG. 21 shows a switchable autostereoscopic display in which an activelens is positioned between an emissive display and an output polariser;

FIG. 22 shows schematically the variation of refractive indices withtemperature for the lens materials;

FIG. 23 shows a control apparatus for optimising lens cell drivingvoltage;

FIG. 24 shows schematically the control of effective lens refractiveindices with temperature for the lens materials for the 2D mode; and

FIG. 25 shows schematically the control of effective lens refractiveindices with temperature for the lens materials for the 3D mode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Some of the various embodiments employ common elements which, forbrevity, will be given common reference numerals and a descriptionthereof will not be repeated. Furthermore the description of theelements of each embodiment applies equally to the identical elements ofthe other embodiments and the elements having corresponding effects,mutatis mutandis. Also, the figures illustrating the embodiments whichare displays show only a portion of display, for clarity. In fact, theconstruction is repeated over the entire area of the display.

In this specification, the direction of the optical axis of thebirefringent material (the director direction, or the extraordinary axisdirection) will be referred to as the birefringent optical axis. Thisshould not be confused with the optical axis of the lenses which isdefined in the usual way by geometric optics.

A cylindrical lens describes a lens in which an edge (which has a radiusof curvature and may have other aspheric components) is swept in a firstlinear direction The geometric microlens axis is defined as the linealong the centre of the lens in the first linear direction, i.e.parallel to the direction of sweep of the edge. In a 2D-3D type display,the geometric microlens axis is vertical, so that it is parallel to thecolumns of pixels of the display. In a brightness enhanced display asdescribed herein, the geometric microlens axis is horizontal so that itis parallel to the rows of the pixels of the display.

In all uses, when reference is made to material being aligned in aparticular direction, there may be a pretilt to prevent degeneracy inthe cell, in which case there remains substantial alignment, althoughnot perfect alignment.

In this document, an SLM (spatial light modulator) includes both ‘lightvalve’ devices such as liquid crystal displays and emissive devices suchas electroluminescent displays and LED displays. In the various displayapparatuses, the pitch of the lenses corresponds to the viewpointcorrection condition, that is the pitch of the parallax barrier isslightly smaller than twice the pitch of the pixel array in order tosteer the light from each pixel to the viewing window.

In the following embodiments, polymer material is used as an isotropicmaterial, but in principle materials other than polymers couldalternatively be used, for example glass, in which case a relief surfacemay be formed by etching.

Zero Twist Active Lenses

An active lens is a lens comprising a switchable birefringent materialwhich allows switching between respective directional distributions. Thefixed lens 94,98 of FIG. 6 may be replaced by an active lens of thepresent invention to advantageously allow switching between for examplea full resolution 2D mode and an autostereoscopic 3D mode.

FIGS. 7 a and 7 b show respective side views of one type of switchablebirefringent lens array, termed an active lens. The lens form comprisesan array of elongate cylindrical lenses. For clarity FIGS. 7 a and 7 bboth illustrate a first cylindrical lens, between electrodes 110 and115, having no control voltage applied across the electrodes 110 and 115and a second cylindrical lens, between electrodes 112 and 114 having acontrol voltage applied across the electrodes 112 and 114. A firstsubstrate 102 and a second substrate 104 have a birefringent material106 sandwiched between them. The first substrate 102 has the surfacerelief structure 108 formed on it. The structure 108 may be comprised ofa substantially isotropic material. Thus the birefringent material 106has a relief surface adjacent the first substrate 102 and the structure108 and has a planar surface adjacent the second substrate 104.

Electrode layers 110 and 112 are formed on the substrate 102, andelectrode layers 114, 115 are formed on the substrate 104. Theelectrodes may for example be transparent electrodes such as Indium TinOxide (ITO). The electrodes 110 and 112 may alternatively be formed onthe surface of the lens structure 108.

The electrodes 110, 112 and 114,115 are shown as separate elements forpurposes of explanation of the effect so that the liquid crystalswitching is shown for different electric fields in different parts ofthe same image. In practical devices, the electrodes on both substratesmay be segmented so that different regions of the lens area can becontrolled independently to be 2D or 3D, or they may be a single elementover the whole display area Specifically the lens array may be passivemultiplexed addressed as is known in the art.

The lenses may be spaced from the second substrate 104 by means ofspacer balls, spacer fibres, spacer ribs or other known spacertechniques. Alternatively, the lens may touch down on to the planarsurface. Advantageously, this removes the need for spacers, but willreduce the active aperture of the lens.

In the gap between the electrodes 110 and 115, the birefringentmolecules are represented by a positive dielectric anisotropy, nematicliquid crystal material with no electric field applied across the cell.The director of the liquid crystal molecules is aligned substantially inthe plane of the surface by means of homogeneous alignment layers 116and 118 at the surface relief structure 108 and second substrate 104. Asmall pretilt (not shown) may be imposed on the cell by the alignmentlayers 116,118. The molecules are represented as elongate ellipses forthe purpose of explanation, with the extraordinary refractive indexparallel to the long axis of the molecule.

In the switched state, the electric field serves to reorient the liquidcrystal molecules so that the director orientation 120 in the middle ofthe lens cell is substantially vertical. This causes a variation in therefractive index profile through the cell.

In operation, such an element is aligned for example on the outputpolariser of a Thin Film Transistor (TFT) LCD, as shown in FIG. 8 a. Ifthe transmission direction 122 of the linear output polarisation of thedisplay is 0 degrees, then the light is incident on a first planesubstrate 124 which comprises the layers 104,114,115 and 118 of thesecond substrate. The alignment direction of the liquid crystal materialat this substrate parallel to the transmission axis of the polariser122. The light passes through the birefringent material and encountersthe lens substrate 126 comprising the surfaces 102,108,110,116.

In the OFF state, no voltage is applied to the cell, and thepolarisation is incident on the extraordinary axis of the liquid crystalmaterial. The refractive index of the polymer is set to be close to theordinary refractive index of the liquid crystal material, and thus thereis a phase difference at the surface of the lens. The lens acts on thelight in the vertical polarisation to cause a change in the directionaldistribution of the optical output. The phase structure can be set sothat the lens has a focal length such that a pixel plane issubstantially imaged to a window plane as well known forautostereoscopic 3D displays and directional viewing systems.

In enhanced brightness viewing systems, an image of the pixel plane isproduced at a window plane for horizontally aligned cylindricalmicrolens arrays 127 rather than vertically aligned lenses. In thiscase, the alignment direction of the output polariser 123 is set to behorizontal as shown in FIG. 8 b and the alignment direction at the plan125 e and surface relief structures 127 are also set to be horizontal.In the following discussion, it will be assumed that the lenses arealigned vertically, but the same apparatus can be applied tohorizontally aligned lenses.

In the ON state, a voltage is applied across the cell and the liquidcrystal material realigns parallel to the electric field, to orientation120 shown in FIGS. 7 a and 7 b so that the incident polarisation statesees substantially the ordinary refractive index of the liquid crystal.As the lens structure 108 is substantially indexed matched to theordinary index of the liquid crystal material, then substantially nooptical effect is observed in the liquid crystal lens. Substantially nomodification of the directional distribution is imparted by the lens inthis mode, and so the directionality of the output is substantially thesame as the output from the base display. In practical systems, theindex matching might not be exact and so, to the extent it is inexact,there may be a small residual optical effect. This mode of operation canbe used for the 2D or unmodified output of the display. Advantageously,this allows a user to see all of the pixels of the display for anautostereoscopic or directional display system, or to see a uniformlyilluminated window plane for an enhanced brightness display system.

FIG. 7 c shows a cell in which the lens cusps are in contact with theplanar substrate and the electrodes are positioned on the surface reliefstructure. Additional insulating layers (not shown) may be incorporatedon the surfaces to prevent electrical short circuits. In general it maybe desirable to incorporate the electrodes on plane surfaces as shown inFIGS. 7 a and 7 b in order to remove electrical shorts, although thismay increase the driving voltage across the cell.

Such a device in which the planar substrate is aligned with the outputpolarisation of the display advantageously operates in the same liquidcrystal mode (for example normally white or normally black) for bothdirectional distributions of operation.

The index matching condition is set so that the directional distributionof the output is substantially unmodified. In practice, there may be asmall difference between the isotropic index and the index of one of thebirefringent material indices. However, for small refractive indexdifferences, the optical output may be relatively insensitive to thedifference, as the size of the eye spot at the pixel plane is similar tothe size of the lens. Therefore, the tolerance on the index matchingcondition may be relatively relaxed. For example, in one fabricatedsystem, the material Norland NOA71 with an isotropic index at 555 nm of1.56 was used in combination with E7 from Merck with a correspondingordinary refractive index of 1.52. In a 2D-3D display demonstration,this led to a small variation of intensity in the window plane in the 2Dmode when used, while not substantially affecting the quality of the 2Dimage.

Twisted Active Lens Cells

In practice, the output polarisation angle of polarised display devicesis not generally set to be vertical. This is due to the optimisation ofthe viewing angle performance of displays such as liquid crystaldisplays. For example, well known normally white transmissive twistednematic displays have output polarisations at an angle of 45 degrees,while transflective and reflective displays have output angles near to20 degrees for example. In the embodiments described so far, the outputangle of the polarisation state on the input and output surfaces of thebirefringent lens are set to be zero which may not match this angle.

The surface relief structure has an associated surface alignment energyand will impart an alignment on to the liquid crystal cell incompetition with the alignment layer. This effect may be particularlyimportant near to the cusps of the lens where the radius of curvature isa minimum. This may cause disclinations in the liquid crystal materialbetween areas of different liquid crystal director orientation.Disclinations may cause scatter, increased optical cross talk, reductionin lens contrast and increased relaxation times and are thereforeundesirable. It is therefore preferable to align the liquid crystalmaterial at the lens surface parallel to the geometric optical axis ofthe cylindrical lenses of the lens array.

One approach would be to produce an alignment layers on the plane andsurface relief surfaces that are parallel to the panel outputpolarisation. This would require an alignment layer at the lens surfacerelief structure which is not parallel to the geometric lens axis of thecylindrical lenses.

In an embodiment of this invention, the output polarisation of thedisplay can be modified by incorporating a waveplate such as a halfwaveplate at the input to the active lens. This enables the outputlinear polarisation state to be rotated to the vertical prior to passingthrough the active lens. Half waveplates and broadband half waveplatesin which chromatic dispersion effects are reduced are well known in theart. The waveplate will have an additional cost due to material andfitting to the device, the waveplate may be chromatic, and the waveplatehas an additional thickness. The separation of the pixel plane and thelens determines the distance of the windows from the display device,therefore increasing this distance increases the distance of the bestviewing zone from the display.

For example, a two view display may have a colour sub-pixel pitch of 80um will have lenses on a pitch of substantially 160 um and a typicalseparation of pixels to lenses of 900 microns comprising a 500 microndisplay substrate thickness, a 200 um output polariser thickness, a 150um glass Microsheet thickness and a 50 um thick liquid crystal layer.This system will produce 65 mm width windows at a distance of 480 mmfrom the display. If a 200 micron thick waveplate is added then thisdistance will increase to 585 mm. Such an increase in nominal viewingdistance is undesirable for many displays, particularly mobile displays.Reducing the pixel size will further increase the viewing distance. Thinwaveplates, such as those made by coating a highly birefringent materialmay be used at the expense of extra processing time and materials.Therefore, introducing an additional waveplate may not be a desirableoption.

It would be desirable to produce an active lens in which the alignmentof the liquid crystal material in the 3D mode at the surface reliefstructure is parallel to the geometric lens axis, but to incorporate thepolarisation rotation function in to the lens device, thus removing theneed for the additional waveplate or for redesign of the display toprovide a vertical output polarisation.

It is a purpose of this invention to provide adjustment of the outputpolarisation in an active lens by using guiding rotation within activelenses, by setting an angle between the alignment of liquid crystalmaterial on the plane substrate and on the lens substrate in the mode inwhich the lens has optical power such that a twist of the incidentpolarisation state occurs through the thickness of the active lens. Theactive lens thus serves to provide a correction of the outputpolarisation from the display device in addition to parallel alignmentof the birefringent material to the geometric optical axis of thecylindrical lenses in the directional distribution modifying mode ofoperation.

This is unexpectedly advantageous, as can be seen by examining anexample system. In the example autostereoscopic 3D display given, usingan isotropic material of refractive index 1.56 and E7 liquid crystalmaterial from Merck Limited with an extraordinary refractive index of1.75 gives an optimum lens radius of curvature of 130 um. Such a lenshas a sag of 27.5 um. A liquid crystal layer of at least this thicknessis thus required at the thickest part of the lens. Such a layer has anoptical thickness of greater than 6 um and from the Gooch and Tarryrelationship is thus in the strongly guided regime, approaching theMaugin limit. Therefore the polarisation state will be convenientlyguided over most of the lens area.

For a positive power liquid crystal lens, the cusps may have a reducedthickness and thus guiding may be less effective. Similarly, for anegative power liquid crystal lens, the centre of the lens may have areduced thickness and the guiding may be less effective. An additionaluniform thickness layer of liquid crystal material may be incorporatednear to the thinnest part of the cell so that the layer continues toguide the polarisation state and thus optimise the contrast of thedevice. An embodiment of the invention is shown in FIG. 8 c for anautostereoscopic display using a base LCD with a 45 degree outputpolariser transmission angle 200. The output from the display isincident on a plane substrate with a homogeneously aligned positivedielectric anisotropy liquid crystal material on its surface. Thealignment layer at the lens surface is parallel to the lenses and thusvertical. A 45 degree rotation of the polarisation state through thelens thus takes place, and the polarisation is incident on theextraordinary axis of the liquid crystal material at the lens surface,creating a phase mismatch and thus lens focal length.

The rotation of the material in the cell may be aided by doping with acholesteric material. To compensate for pre-tilt effects near to thealignment layers, the alignment direction at the plane and surfacerelief surfaces may have a resolved alignment direction component in thevertical direction that is anti-parallel, so that the cell has asubstantially uniform pre-tilt through the structure in the OFF state.Optionally the cell may have components of alignment in the verticaldirection which are parallel or anti-parallel in which case the responsespeed may be improved.

In the ON state, the molecules are substantially driven in to thevertical alignment state as shown in FIG. 7 b, with molecular alignment120 parallel to the applied field. In this case, there will be reducedrotation of polarisation in the lens so that the polarisation stateincident on the lens surface is not parallel to the geometric lens axis.However, the polarisation state will be aligned to the ordinarycomponent of the liquid crystal refractive index, and thus the lenseswill be substantially index matched, and will have substantially nooptical power.

In the ON state, there may be residual tilt of the liquid crystalmaterial near the alignment layers, which will cause an increase in thefocal length of the lens for the incident polarisation state. This tiltcan be compensated by reducing the refractive index of the ordinarycomponent of the liquid crystal material so that it is lower than theisotropic material. The bulk focal length of the lens can thus becompensated. The radius of curvature is set from the extraordinary indexof the liquid crystal material to determine the focal length in the OFFstate.

In all of the embodiments of this invention, the alignment of thealignment layer on the first substrate may be orthogonal to rather thanparallel with the input polarisation state from the polarised displaydevice. This may advantageously improve the guiding rotation of thepolarisation in the active lens cell in the first mode of operation.This is illustrated in FIG. 8 d in which the alignment direction at theplanar substrate 203 is orthogonal to the panel output polarisation 200.The lens array has an alignment direction 205.

FIG. 18 shows another embodiment of the invention for use in an enhancedbrightness transflective display mode. A light source 252 illuminates anactive lens comprising a support substrate 254 with an ITO coating 256,and an isotropic microstructure 258. Alignment layers 260 are formed onthe surface of the isotropic structure 258 and a liquid crystal material262 is sandwiched between the alignment layer 260 and a planar substrate268 with ITO coating 266 and alignment layer 264 formed on its surface.The substrate 266 may be Microsheet of thickness 160 microns forexample. The active lens is placed on a polariser and waveplate stack270 as used in a standard transflective display. The counter substrate272 and liquid crystal layer 274 are formed on a transflectivereflective pixel plane 276 with absorbing regions 277, reflectiveregions 278 to reflect incident light and transmissive regions 280 totransmit light from a backlight 282 and an active matrix backplane 284.

Light from the ambient light source 252 (not part of the apparatus)illuminates the display. In a first mode of operation, the lenses are inthe OFF state so that there is a focussing function. The light source isthus focussed on to the pixel plane 276. The reflected light iscollected by an adjacent lens where it is focussed to an observer asshown by the rays 286. In limited windows in front of the display, theimage will appear brighter. The overall brightness is conserved, as thebright windows are interspersed by darker windows. In transmissive mode,light from the limited regions of transmission are likewise focussed toa window plane to increase the apparent brightness of the image. Frontalreflections 288 are in a different direction to the useful light 286 andare therefore not seen The operation of the display of FIG. 18 is shownin FIG. 19. The incident light is resolved in to two polarisation statesat the lens 262. In the OFF state, the polarisation state that isfocussed by the lens is parallel to the lens geometric axis 292 and isrotated through the lens on to the planar substrate 294 such that it isincident parallel to the transmission axis of the polariser 295 of thedisplay, which for a typical transflective display may be at 20 degreesfor example. This light is transmitted on to the reflective backplane296 where it is modulated by the display and reflected through thepolariser 298 and planar substrate 300. A second rotation of the lighttakes place so that it is focussed towards an observer 290 by the lens302.

In the non-lensing mode, the ON state liquid crystal alignment is suchthat the incident polarisation sees the ordinary liquid crystal index inall polarisations, so the lens has effectively no function and theoptical output is substantially unmodified.

A further advantage of the use of non-zero twist in the active lensdevice is that the alignment degeneracy of the cell can be reduced.Degeneracy arises from multiple minimum energy twist directions of theliquid crystal in the cell for liquid crystal alignment between theupper and lower surfaces. If there are multiple rotation energy minima,then there are multiple rotation paths the molecules can follow throughthe cell, and therefore the guiding effects can be different, givingdifferent lens properties in different parts of the cell. Using arotation offset allows a single preferred rotation path for themolecules in the cell, and thus increases cell uniformity.

In this way, a rotation of the polarisation state in the active lensserves to optimise the lens performance for the particular panel whilemaintaining parallel alignment at the lens surface. The device works inthe same way for the transmitted light as the reflected path of thereflected light.

In a further embodiment, shown in FIG. 20, the cylindrical lens arraymay be tilted with respect to the columns of pixels of the panel.Tilting the lens axis with respect to the pixels of the panel is wellknown in the art in order to increase the apparent number of views, withthe penalty of increased cross talk between the views. In the presentinvention, it is desirable for the active lens to have an alignment ofliquid crystal at the lens surface which is parallel to the geometriclens axis. Such a device may advantageously use twist between the planarand surface relief surface in order to achieve the twist. FIG. 20 showsa pixel plane 304 in which the pixels 306 are arranged in columns androws. The pixels are incorporated in a transmissive normally whitetwisted nematic liquid crystal display with a 45 degree outputpolarisation 308. A planar substrate 310 of an active lens has analignment direction at 45 degrees, and a surface relief lens 312 has analignment of geometric lens axis at 10 degrees for example. Thealignment layer direction on this surface 312 is set at 190 degrees sothat the alignment directions on the planar substrate have antiparallelcomponents in the vertical direction.

The lenses may alternatively be oriented with a the opposite curvatureat the surface relief plane to that shown in the figures. In this case,the thinnest part of the liquid crystal lens is aligned at the centre ofthe lens rather than near to the cusps. Such a lens suffers from reducedquality wavefront due to increased aberrations of the lens.

Homeotropic Alignment Layers

The embodiments described thus far use conventional homogeneousalignment layers, and positive dielectric anisotropy materials. However,in order to operate the device in 2D mode, a voltage is required to beapplied across the cell. The 2D mode in many devices is likely to beused more. Operation of the device in 3D mode for 2D images will causeunwanted artefacts to users of the display and thus is undesirable.Therefore, such an element will be required to be maintained mostly inits switched state of operation. This will cause increased powerconsumption of the device compared to the 3D mode. If the switch fails,the device will remain in the 3D mode, which may also be undesirable.

In order for the device to operate in 2D mode when in the OFF state, therefractive index of the optical microstructure material 108 could be setto be the same as the extraordinary refractive index of the liquidcrystal material. This is undesirable due to the choice of polymer andliquid crystal materials readily available at low cost and which aresafe to handle.

In the above example, E7 is a typical positive dielectric anisotropymaterial from Merck, with extraordinary refractive index (n_(e)) of 1.75with a delta n of 0.22 at 550 nm. Polymer materials are available withrefractive indeces greater than 1.6, but these materials tend to betoxic, expensive and difficult to handle and therefore are undesirable.Alternatively, the n_(e) of the positive dielectric anisotropy liquidcrystal material can be reduced to match more acceptable polymermaterials. However, reducing the n_(e), tends to reduce the delta n ofthe material. For example, MLC3376 from Merck Limited has an n_(e) of1.57, but a delta n of only 0.09. Such a lens would require a lenscurvature of less than—100 microns. Such a lens thus has an increasedsag, reversed orientation, increased surface reflections and degradedaberrations leading to increased scatter, optical cross talk, andresponse time. Such a lens will also be less strongly guiding and so therotation of input polarisation in the cell will be less effective.

Therefore, for many polymers typically used to form the lens structure,it is simpler, cheaper and provides higher performance to use an indexmatch of the isotropic material to the ordinary rather thanextraordinary component of the refractive index of the liquid crystalmaterial.

Thus, active lens devices fabricated using a combination of homogeneousalignment, positive dielectric anisotropy liquid crystals and readilyavailable polymer materials will in general require a driven 2D mode ofoperation which is undesirable for the reasons described above.

An active lens is configured as shown in FIGS. 9 a and 9 b which issimilar to FIG. 7. The substrate 102 has electrodes 110,112, an opticalmicrostructure 108, and a homeotropic alignment layer 128 formed on itssurface, while the substrate 104 has electrodes 114,115 and homeotropicalignment layer 130 formed on its surface. The cell is filled with aliquid crystal material 132 with a negative dielectric anisotropy.

In operation of the device is described in FIG. 10 a. The outputpolariser of the display provides an input linear polarisation statedirection 136 parallel to the vertical. Homeotropic alignment causes thedirector to align substantially orthogonal to the plane of the surface.In the OFF state, as shown by the liquid crystal material orientation124 under electrode 110 in FIG. 9, the polarisation state is incident onthe liquid crystal material, and sees the ordinary refractive index ofthe liquid crystal material, which in turn is matched to the refractiveindex of the isotropic polymer microstructure 108. Thus, there is nosubstantial phase step at the lens, and the lens has substantially noeffect on the output directional distribution.

In the ON state, as shown under the electrode 112 in FIGS. 9 a and 9 b,the negative dielectric anisotropy of the liquid crystal causes itsorientation 132 to be modulated so that the director in the middle ofthe cell is substantially in the plane of the substrates. There istherefore a component of the extraordinary index of the liquid crystalseen by the output polarisation of the display, and a phase step isproduced at by the lens. Such a lens can be used to alter thedirectional distribution of the display and create for example a 3D modeof operation.

Such an embodiment advantageously serves to create an unswitched 2D modeof operation using conventional lower index polymer materials and isthus cheaper and simpler to manufacture. In this embodiment, the powerconsumption of the device is only present when the device is in the 3Dmode of operation.

In the ON state, the liquid crystal molecules are pulled parallel to thesubstrates by the electric field. The bias of the alignment of theliquid crystal material at the lens surface may be made parallel to thegeometric microlens axis for example by the surface energy of the lensmicrostructure. Additional alignment features may be incorporated onthis surface so as to promote alignment in this orientation in thedriven state. Such alignment features may be grooves running parallel tothe geometric microlens axis formed in the polymer microstructure. Suchgrooves may be formed by diffraction gratings for example. Thediffraction gratings may be formed in the mastering process of themicrolens structure, so that they advantageously can be formed in thesame replication process as the lenses.

The planar substrate may also have a homogeneous alignment structure tobias the orientation of the molecules in the ON state. The bias of thealignment layer in the ON state may be arranged to provide rotation ofthe polarisation through the cell so that the output polarisation statefrom the display device is rotated to be substantially parallel to thegeometric microlens axis. This is illustrated in FIG. 10 b. A paneloutput polarisation of 45 degrees for example is incident on the planesubstrate. In the OFF state, a homeotropically aligned liquid crystal isseen with ordinary refractive index matched to the isotropic materialand thus substantially no rotation is required. In the ON state, though,the alignment layer 208 has a homogeneous alignment bias so that theliquid crystal molecules are aligned substantially parallel (ororthogonal) to the output polarisation direction of the panel. At thelenses, the homogeneous alignment bias is anti-parallel to the verticalcomponent of the alignment at the plane substrate and thus a rotation isprovided through the cell. Such a rotation provides the same advantagesas described above, in particular allowing the alignment of the lenseswith the output polarisation of a standard display without therequirement for additional waveplates or other modification of thedisplay device output polarisation, thus maximising the viewing angle ofthe device.

A further embodiment of the invention is shown in FIGS. 11 and 12. Inthis configuration, the alignment layer 134 at the planar substrate is ahomogeneous alignment layer, causing the liquid crystal to orientparallel to the substrate. In the OFF state, the incident polarisationsees the extraordinary index in the material 136 close to the planesubstrate. However, the material 138 close to the lens surface isoriented homeotropically, so that the polarisation state seessubstantially the ordinary index in the surface relief region. As thepolymer refractive index is substantially matched to the ordinaryrefractive index of the liquid crystal material, substantially no phasestep is present and the lens has no function.

To compensate for the additional power of the lens in the region nearthe homogeneous alignment layer, the ordinary refractive index of theliquid crystal may be lower than the index of the polymer material. Thecurvature of the lens is set by the extraordinary index of the liquidcrystal material and the polymer index.

In the ON state, the negative dielectric anisotropy material reorientsso that the director 140 is substantially parallel to the substratesthrough the entire cell, and the polarisation state sees theextraordinary index of the liquid crystal material. A phase step at thelens is then present, giving rise to a modified directional distributionand a 3D mode of operation. This is illustrated by the directororientation 140 under electrodes 112 in FIGS. 11 a and 11 b.

The combination of homeotropic and homogenous alignment at therespective alignment layers on the lens surface may be used to form abistable lens as shown in FIG. 13. Such a lens does not require power tobe applied other than to switch between the two states. Planar cells forpixel intensity switching applications have been described in “Gratingaligned bistable Nematic device”, G. P. Bryan-Brown, C. V. Brown, J. C.Jones, E. L. Wood, I. C. Sage, P. Brett, J. Rudin SID 97 Digest pp37-40. A combination of a grating and homeotropic alignment layer areused to create a bistable cell.

In the first electrode region 110, a pulse is driven across the cell sothat the material aligns in accordance with the homeotropic alignmentlayer across the cell. The incident linear polarisation state then seesthe ordinary index of the liquid crystal material 146, and no lensstructure is resolved. If a dc voltage pulse is applied then the liquidcrystal alignment at the lens surface is modified so that the director148 lies parallel to the grating surface. Thus in the region of the lenssurface, the polarisation state sees the extraordinary index of theliquid crystal material and the lens is resolved. If the surface of theenergy of the grating is set to be similar to the surface energy of thehomeotropic alignment layer then the device may be bistable and thedevice will be undriven in both 2D and 3D modes. The device is switchedbetween 2D and 3D modes by application of a positive or negative voltagepulse.

In the above embodiments a homeotropic alignment layer is used at thelens surface so that in the first undriven mode, for the incidentpolarisation state sees the ordinary refractive index of the liquidcrystal which is index matched to the polymer. By using the ordinaryrefractive index, standard polymer materials may be used in devicefabrication. As the phase structure is produced at the lens surface, theplane surface of the lens may use either homeotropic or homogeneousalignment.

External Polariser Active Lens Device

Prior art displays do not disclose the relative position of the activelens with respect to the polariser in a polarised output display system.As shown in FIG. 14, the active lens may comprise a substrate 211, whichmay for example be a Microsheet glass or a plastics material substrateof thickness 150 microns or less, planar substrate alignment layer andITO coating 212 and lens substrate alignment layer and ITO coating 214,birefringent material 218 and isotropic material 220 and final substrate216. The structure may be placed following the output polariser of thedisplay. The intensity has been analysed by the display polariser, andthus the lens manipulates directionality only of the output light. Sucha system suffers from increase in viewing distance due to the outputpolariser thickness.

The viewing distance of the system can be reduced by positioning thepolariser after the active lens of the device as shown in FIG. 15. Inthis case, the LCD polarisation has yet to be analysed by the outputpolariser 82 but passes through the active lens 211-220 first. Theoperation of the display is shown in FIG. 16. For example, if thedisplay is a liquid crystal display with normally white outputpolarisation angle 222 set at 0 degrees, by means of an outputpolariser, the planar substrate has a homogeneous alignment with analignment angle 224 of 0 degrees. In the lens OFF state, the white statelight from the LCD passes through the lens so that the polarisation 226incident on the cylindrical lens array is parallel to the geometric lensaxis. This light is then transmitted through the output polariser 228with a transmission direction at 0 degrees. In the lens ON state, anelectric field is applied to the cell and the positive dielectricanisotropy material is realigned to be substantially orthogonal to thesubstrates of the cell. The polarisation state thus sees the ordinaryrefractive index of the lens and no lens function is imparted. Theoutput polarisation state passes through the output polariser.

Such a configuration will not conveniently operate in devices with nonzero degrees polarisation rotation in the lens cell due to loss ofcontrast at the output polariser. They may require the addition of oneor more waveplates in order to rotate the output polarisation of thedisplay. Waveplates can advantageously be made thinner than polarisers.Contrast loss in the lens cell may also serve to degrade the contrast ofthe final image.

The external polariser embodiment additionally has the advantage thatthe visibility of the lenses in external ambient light is reduced.External light sources incident on the front of the display pass throughthe input polariser, undergo Fresnel reflections at the lens and othersurfaces with phase steps, (for example from reflective coatings such asITO) and then pass back through the output polariser. Therefore, theexternal polariser absorbs a proportion of the light passing in eachdirection, and thus reduces lens reflections, which advantageouslyincreases display contrast.

Advantageously, such elements can be used with liquid crystal modes withhigh polarisation conversion efficiency to optimise the contrast of themode. In some devices, such as transmissive normally white twistednematic liquid crystals, the ON state (referring to the polarisationstate which produces the maximum white level) has a 90 degree rotationwith respect to the black state. Thus the polarisation state passingthrough the lens can be resolved from just the ON state. In otherdevices, such as mixed twisted nematic devices for example, the ONpolarisation state may not be orthogonal to the black state. Suchdevices may suffer from reduced contrast in the internal active lensconfiguration.

In systems in which waveplates are used on the optical output inaddition to polarisers, such as in many reflective liquid crystaldisplays, the waveplates may be positioned between the pixel plane andthe active lens device so that the output from the display issubstantially linear.

FIG. 21 shows a further embodiment of the invention. An emissive displaysuch as a polymer electroluminescent display for example comprises asubstrate 314 on which emissive pixels 316-330 are formed. The emissivepixels may be non-polarised, partially polarised or polarised with alinear output polarisation direction which is aligned to provide optimumtransition through the remainder of the elements of the display. Anemissive display counter substrate 332 has an active lens elementattached. The active lens comprises for example a Microsheet 334, ITOelectrodes 336,342, a switchable birefringent material 338 an isotropicsurface relief structure 340, and a support substrate 344. A finaloutput polariser 346 is attached with a transmission direction which isparallel to the geometric axis of the microlens array.

In the OFF state, the liquid crystal material receives the light fromthe pixels and for the vertical polarisation state (out of the plane ofthe paper), a phase mismatch at the refractive structure is generated,so the lens has an optical function. This polarisation state istransmitted through the output polariser 346. Thus the active lens maybe positioned between the pixel plane and the output polariser,advantageously reducing the viewing distance of the display. In the ONstate the molecules realign so that the output polarisation that istransmitted through the polariser 346 has seen the ordinary refractiveindex of the liquid crystal material 338, and no lens function is seen.In this way the polariser 344 serves to combine the functions ofclean-up polariser and lens analysing polariser.

Polarised Emissive Display

Emissive displays such as inorganic and organic electroluminescentdisplays including polymer and small molecule organic electroluminescentdisplays typically produce an unpolarised optical output. However,directional distribution optical switching systems may rely onpolarisation switching in order to enable a display to be reconfiguredbetween a first mode which may be Lambertian for example, and a secondmode which may be autostereoscopic 3D windows for example. Unpolariseddisplays will thus show a polarisation loss when combined withpolarisation directional distribution optical switching systems.

It is the purpose of this invention to provide high optical efficiencyin emissive displays by aligning the output polarisation of polarisedemissive displays with the input polarisation state of directionaldistribution optical switching systems comprising active lenses. Thepolarisation alignment may be achieved by means of uniaxial alignedchromophores of the emissive material in the emissive pixels of thedisplay. The alignment direction of the major axis of the polarisationoutput may be set to cooperate with the alignment directions of thebirefringent material in a birefringent microlens.

In this way, a high efficiency emissive directional distribution opticalswitching display using active lenses may be achieved. Such a displayhas additional advantages over LCD displays, for example not requiring abacklight and thus can be made thinner and lighter which can beimportant for mobile applications.

Any type of polarised emissive display which provides a polarised outputmay be used. For example, it may be the polarised organicelectroluminescent display described in “Polarized Electroluminescencefrom an Anisotropic Nematic Network on a Non-contact PhotoalignmentLayer”, A. E. A Contoret, S. R. Farrar, P. O. Jackson, S. M. Khan, L.May, M. O'Neill, J. E. Nicholls, S. M. Kelly and G. J. Richards, Adv.Mater. 2000, 12, No. 13, July 5 p 971. This demonstrates thatpolarisation efficiencies of 11:1 can be achieved in practical systems.

FIG. 17 shows an embodiment of the present invention. An array of pixels230-244 is formed on a display substrate 246. The substrate 246 maycomprise an array of addressing thin film transistors and electrodes sothat each of the pixels may be independently addressed with anelectrical signal. The thin film transistors may be inorganic or may beembodied in organic materials. Alternatively, the pixels may beaddressed by a passive addressing scheme in which addressing transistorsneed not be present at the pixels. Each of the pixels 23-244 comprisesan emissive region in which the emissive material comprisingchromophores is uniaxially aligned so that the polarisation of emissionis substantially linear and substantially in the same orientation forthe entire pixel. Each pixel is arranged to have substantially the samepolarisation direction The emissive material may be a polymerelectroluminescent material or a small molecule electroluminescentmaterial for example. Means to produce polarised emission by aligningthe molecules of the emitting material are known. A further coversubstrate 248 is attached to the pixels. The substrate 248 mayincorporate barrier layers and contrast enhancement black mask layers.

An optional polariser 250 may be attached to the substrate 248.Alternatively, polariser materials may be incorporated at or near to thepixel plane, on the inner surface of the substrate 248 for example.

For an example, one known polarised organic electroluminescent displayhas a polarisation ratio of 11:1. In combination with a typicalpolariser of polarisation efficiency 45%, the overall throughput fromthe light source will be 82.5%, compared to 45% for an unpolarised lightsource in combination with a clean-up polariser.

The active birefringent microlens 212-220 is formed on the surface ofthe polariser 250. In order to switch the output polarisation from theswitch cell, a voltage is applied across the liquid crystal cell.

The apparatus of FIG. 17 operates in the following manner. The outputpolarisation from the polarised emissive pixel array 230-244 is cleanedby the linear polariser 250 which has a transmission direction parallelto the major axis of the polarisation direction of the emissivematerial. This polarisation state is aligned parallel to the alignmentof the liquid crystal material in the birefringent lens 218 in its OFFstate. This refractive index is different to the refractive index of theisotropic material 40, and thus there is a lens effect. In a secondmode, the material 218 is realigned by the applied field so that thereis substantially no index step to the isotropic material at the lenssurface, and the lens has no optical function. This causes a change inthe directional distribution of the optical output to the 2D mode. Thelens may be arranged to produce an image of the pixel plane at a windowplane.

Extending the Range of Optimum Operating Temperature

Considerations to extend the range of optimum operating temperature willnow be described. These considerations apply to all the activebirefringent lens arrays described above and indeed apply in general toany other active birefringent lens array operable in two modes bycontrol of the voltage across electrodes.

The performance of the display may change with the operatingtemperature. This can be due to a variation in the ordinary andextraordinary refractive indices of the birefringent material and therefractive index of the isotropic material with temperature.

The variation of refractive index 350 against temperature 352 for atypical combination of liquid crystal and polymer material is shownschematically in FIG. 22. The ordinary index 356 tends to increase asthe nematic to isotropic transition temperature is approached while theextraordinary index 354 decreases. Above the nematic-isotropictransition temperature 360, the birefringent material indices becomematched. The polymer index 358 may decrease with temperature, as shown.

The system shown in FIG. 22 shows the case of a typical material systemin which the polymer index 356 is substantially matched to the ordinaryindex of the birefringent material for a positive dielectric anisotropyliquid crystal material. As described elsewhere in the presentapplication, such a system typically requires a voltage to be applied tothe cell to enable the 2D mode of operation in which the polymer andordinary indices are substantially matched. In other material systems,the polymer index 356 may be substantially matched to the extraordinaryindex of the birefringent material, in which case the followingconsiderations still apply mutatis mutandis.

The design operating temperature 362 is typically room temperature forexample in the range of 20-25° C., and preferably 20° C.

The zero voltage index match point 368 at which the polymer index isequal to the ordinary index 356 may be chosen as a design parameterdepending on the precise choice of materials. Typically the zero voltageindex match point 368 is set at the design operating temperature.However, it has been appreciated that there is advantage in biassing thezero voltage index match point 368 to a higher temperature, as follows.

Firstly, there is a consideration that display apparatuses are used moreoften at temperatures above the normal design temperature of 20° C.Therefore raising the temperature at which the refractive index of thepolymer material is exactly equal to the relevant one of the refractiveindices of the birefringent material actually causes the refractiveindex of the polymer material to be closer to the relevant one of therefractive indices of the birefringent material over a greaterproportion of the typical use of a display apparatus.

For a system in which the ordinary index 356 of the birefringent lensmaterial is substantially matched to the polymer index 358 at the designtemperature 362, the index matching condition may be lost as thetemperature increases. This can be overcome by setting the polymer indexbetween the ordinary and extraordinary indices at room temperature suchthat an acceptably low variation of intensity is seen at the windowplane for the 2D mode. As the operating temperature rises, the liquidcrystal ordinary index 356 rises towards the polymer index 358 so thatthe range of temperatures over which the a sufficiently close indexmatching condition to meet the 2D lens performance requirement isextended, for the typical range of desirable operating temperatures, asdescribed above.

Secondly, there will now be described an apparatus, as shown in FIG. 23,which can be used to further compensate the performance of the devicefor variations in temperature. A temperature sensor 370 or a manual useradjustment 372 is used to set a voltage controller 374 which drives thevoltage across the lens cell 376. A small voltage can thus be appliedacross the lens in the 2D mode. The manual user adjustment may beimplemented through a direct electronic adjustment or through user inputto a software application which controls the voltage.

The polymer index 358 is set between the ordinary refractive index 358and the extraordinary refractive index 354 over a range of temperaturesup to a limit which limit is above the design operating temperature 362,preferably at least 25° C. In this particular embodiment, the limit isthe zero voltage index match point 368. Over this range of temperatures,the voltage controller 374 can compensate for the variation intemperature in the second mode (2D mode), thereby extending theeffective temperature operating range.

FIG. 24 shows schematically the variation of refractive index in such adevice. The effective refractive index is the resultant refractive indexseen by the polarisation state passing through the birefringent lens tothe observer. The effective refractive index is the resolved componentof the ordinary and extraordinary refractive indices of the birefringentmaterial. If a voltage is applied to the birefringent material, thematerial reorients so that the relative components of the ordinary andextraordinary refractive indices seen by the polarisation state varies,and thus the effective index varies.

In the example given, at the design operating temperature 362, theeffective refractive index seen by the incident polarisation state as itpasses through the lens is lower than the polymer index if a voltage isapplied to fully switch the birefringent material.

If a reduced voltage is applied, the effective index seen by thepolarisation will increase by an amount shown by the arrow 364, as thepolarisation state starts to see a component of the extraordinary indexof the birefringent material. If the polymer index 358 is set above theordinary index of the liquid crystal at the standard operatingtemperature then the effective index in the lens can thus be controlledto match the polymer index 358 at the operating temperature.

If the voltage is adjusted to match the operating temperature across thetemperature range, for example at the temperature shown by the arrow366, then the operating temperature range of the second mode can beextended. At the temperature 368, no voltage is applied to achieve theindex matching condition. The voltage signal 374 can be set by automaticfeedback from a temperature sensor 370, or by manual correction by input372 to optimise the display performance.

Similarly, the operating temperature range in the first mode (3D mode)can be extended, as illustrated in FIG. 25. As the temperature 352increases, the extraordinary index 354 of the material will reduce, thusreducing the lens optical power. The power of the lens array, which maybe controlled by selection of the radius of curvature of the lenses, maybe set to be greater than the power required for design temperature 362operation at best focus of the spatial light modulator in the displayapparatus over a range of temperatures up to a limit which limit isabove the design operating temperature 362, preferably at least 25° C.Over this range of temperatures, the voltage controller 374 cancompensate for the variation in temperature in the first mode (3D mode),thereby extending the effective temperature operating range.

In this particular embodiment, if no voltage is applied to the cell, theoptical power of the lens is slightly higher than is required foroptimum performance at room temperature. An adjusted voltage can beapplied across the lens cell to achieve optimum lens operation. This isillustrated by the arrow 370, in which the effective index seen by thepolarisation state in the lens falls to a locus of refractive index 372.As the temperature increases, the effective index in the lens will fall,so that the refractive index drop 374 required is smaller, and thedriving voltage can be reduced to maintain the optical performance ofthe lens. The same control system as shown in FIG. 23 can be used tooptimise the performance of the system by setting the best displayfocus. A calibration of best focus across the temperature range can bedetermined for the display prior to shipment to the end user.

The best focus of the lens is determined by the lens optical power andthe separation of the lens from the pixel plane of the spatial lightmodulator.

The best focus of the lens can be defined for example as well known inthe art as the paraxial focus, the minimum on-axis spot size, the fieldaveraged spot size, the minimum on-axis root mean squared optical pathdifference, or the minimum field averaged root mean squared optical pathdifference. Alternatively, the best focus may be determined with respectto the size of the eye spot at the pixel plane (i.e. the size of theimage of the nominal human pupil at the pixel plane). The best focus cangenerally be set by minimising the size of the eye spot. Alternativelythe best focus may be different from the minimum eye spot size at thepixel plane. For example, the best focus eye spot size may be largerthan the gap between the pixel columns so that the resultant spot servesto blur out intensity differences in the window plane due to imaging ofthe gaps between the columns. The best focus can alternatively bedetermined by visual observation of the display, so that the visualappearance of the fringe seen on the display surface as the observermoves laterally with respect to the display is optimised for bestperceived appearance of the display. The best focus setting may be auser setting on the display, to allow users to vary the displayappearance to best meet their personal preferences.

If the lens incorporates homeotropic alignment layers and a birefringentmaterial with negative dielectric anisotropy, then the larger drivingvoltage is required for 3D operation. In this case, the 2D mode isoptimised by increasing the driving voltage so that the effective indexincreases towards the polymer index. In the 3D mode, the driving voltageis reduced so that the effective index falls to obtain the best focusposition.

The polymer index may also be set to be close to, but less than, theextraordinary index of the birefringent material. In this case, a smallchange in the effective index will produce the index match, whereas areduction in the drive voltage can be used to increase the effectiveindex in the 3D mode and so produce the index step for best focus.

In this way, the operating range of the display can be advantageouslyextended by setting the polymer index between the ordinary andextraordinary refractive indices.

Additionally, the operating temperature range of the 3D mode may beadvantageously optimised by setting the lens radius of curvature to besmaller than for a corresponding lens optimised at the design operatingtemperature.

In the embodiments of the invention in which the lens incorporates twistmay require further compensation. The amount of twist in the lens may bedetermined by the offset drive voltage. Thus, a small offset drivevoltage may cause less twist in the lens than is present for no drivevoltage. Alternatively, an offset of the maximum drive voltage mayintroduce twist that was not otherwise present. To remove the effects oftwist, it may be desirable to use a waveplate to compensate for paneloutput polarisation direction. Alternatively, the design twist in thedevice could be set at manufacture to be optimised so that the correctresultant twist occurs in the lens cell at the design operatingtemperature when the offset voltage has been applied.

1. A display apparatus comprising: a display device having a spatiallight modulator and an output polariser; and an electrically switchablebirefringent lens array arranged to receive light from the spatial lightmodulator, wherein the lens array is arranged between the spatial lightmodulator and the output polariser of the display device.
 2. A displayapparatus according to claim 1, wherein the spatial light modulator isarranged to rotate the major axis of the polarisation direction of lightat each pixel by a modulated amount and the output polariser is arrangedto select light polarised in a predetermined direction.
 3. A displayapparatus according to claim 2, wherein the spatial light modulator is aliquid crystal spatial light modulator.
 4. A display apparatus accordingto claim 1, wherein the lens array is electrically switchable between afirst mode in which the lens array modifies the directional distributionof incident light polarised in a predetermined direction and in saidsecond mode to have substantially no effect on incident light polarisedin said predetermined direction.